Define radioactive decay constant.

Explanation

**Step-by-Step Solution:**

1. **Definition of Radioactive Decay Constant:**
The radioactive decay constant, often denoted by the symbol \( \lambda \), is a measure of the probability of decay of a radioactive substance per unit time. It quantifies how quickly a radioactive isotope will decay.

2. **Mathematical Representation:**
The decay constant is defined as the reciprocal of the time during which the number of atoms in a radioactive substance reduces to approximately 36.8% of its initial amount. This percentage corresponds to a time period known as the mean lifetime of the radioactive substance.

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